The present invention relates to exposure indicators or monitors and, more particularly, to an exposure indicating electronic circuit for use in conjunction with photographic plate or film making equipment and the like.
Double exposures and wasted film therefrom because of distractions and the like tending to make the operator forget the fact that a particular plate has already been exposed has been a longstanding problem in the plate making art.
One solution to this problem is evidenced by the circuit in prior U.S. Pat. No. 4,111,549 wherein there is disclosed an exposure indicator which is attached to the hinged glass mask or contact frame of a plate making apparatus and includes a light emitting diode and a light sensitive SCR. When illuminated by the plate maker light source, the SCR resistance drops thereby causing the light emitting diode to glow; indicating that an exposure has taken place. According to this prior patent, the SCR and light emitting diode circuit is placed in readiness for operation by means of a switch responsive to movement of the glass frame into contact with the film to be exposed, which is intimately sandwiched between the glass and a lower bed. Although a variable resistance is provided in an attempt to vary the sensitivity of the SCR to various illumination levels, the nature of the light sensitive SCR does not permit extremely wide variations.